Free Knowledge Resources


Linux Manual Pages


Free Software * Books

Source Code

Free Media

Linux

kill(2)


NAME

   kill - send signal to a process

SYNOPSIS

   #include <sys/types.h>
   #include <signal.h>

   int kill(pid_t pid, int sig);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

   kill(): _POSIX_C_SOURCE

DESCRIPTION

   The  kill()  system  call can be used to send any signal to any process
   group or process.

   If pid is positive, then signal sig is sent to the process with the  ID
   specified by pid.

   If pid equals 0, then sig is sent to every process in the process group
   of the calling process.

   If pid equals -1, then sig is sent  to  every  process  for  which  the
   calling  process  has  permission to send signals, except for process 1
   (init), but see below.

   If pid is less than -1, then sig  is  sent  to  every  process  in  the
   process group whose ID is -pid.

   If  sig  is  0,  then  no  signal is sent, but existence and permission
   checks are still performed; this can be used to check for the existence
   of  a  process  ID  or process group ID that the caller is permitted to
   signal.

   For a process to have permission to send a signal, it  must  either  be
   privileged  (under  Linux:  have  the  CAP_KILL  capability in the user
   namespace of the target process), or the real or effective user  ID  of
   the  sending  process  must  equal the real or saved set-user-ID of the
   target process.  In the case of SIGCONT, it suffices when  the  sending
   and receiving processes belong to the same session.  (Historically, the
   rules were different; see NOTES.)

RETURN VALUE

   On success (at least one signal was sent), zero is returned.  On error,
   -1 is returned, and errno is set appropriately.

ERRORS

   EINVAL An invalid signal was specified.

   EPERM  The  process  does not have permission to send the signal to any
          of the target processes.

   ESRCH  The process or process group  does  not  exist.   Note  that  an
          existing   process  might  be  a  zombie,  a  process  that  has
          terminated execution, but has not yet been wait(2)ed for.

CONFORMING TO

   POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

NOTES

   The only signals that can be sent to process ID 1,  the  init  process,
   are  those  for  which  init  has explicitly installed signal handlers.
   This is done to assure the system is not brought down accidentally.

   POSIX.1 requires that kill(-1,sig) send sig to all processes  that  the
   calling   process  may  send  signals  to,  except  possibly  for  some
   implementation-defined system processes.  Linux  allows  a  process  to
   signal  itself,  but on Linux the call kill(-1,sig) does not signal the
   calling process.

   POSIX.1 requires that if a process sends a signal to  itself,  and  the
   sending  thread  does  not have the signal blocked, and no other thread
   has it unblocked or is waiting for  it  in  sigwait(3),  at  least  one
   unblocked  signal  must  be  delivered to the sending thread before the
   kill() returns.

   Linux notes
   Across different kernel versions, Linux has  enforced  different  rules
   for  the  permissions  required  for  an unprivileged process to send a
   signal to another process.  In kernels 1.0 to 1.2.2, a signal could  be
   sent  if  the effective user ID of the sender matched effective user ID
   of the target, or the real user ID of the sender matched the real  user
   ID  of  the  target.  From kernel 1.2.3 until 1.3.77, a signal could be
   sent if the effective user ID of the sender matched either the real  or
   effective  user  ID of the target.  The current rules, which conform to
   POSIX.1, were adopted in kernel 1.3.78.

BUGS

   In 2.6 kernels up to and including 2.6.7, there was a  bug  that  meant
   that  when  sending  signals to a process group, kill() failed with the
   error EPERM if the caller did not have permission to send the signal to
   any   (rather   than   all)  of  the  members  of  the  process  group.
   Notwithstanding this error return, the signal was  still  delivered  to
   all of the processes for which the caller had permission to signal.

SEE ALSO

   kill(1),    _exit(2),    signal(2),   tkill(2),   exit(3),   killpg(3),
   sigqueue(3), capabilities(7), credentials(7), signal(7)

COLOPHON

   This page is part of release 4.09 of the Linux  man-pages  project.   A
   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at
   https://www.kernel.org/doc/man-pages/.



Opportunity


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.



Free Software


Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.



Free Books


The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.



Education


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.


birds banner